Colloidal dispersions such as emulsions and suspensions are dispersed systems consisting of two or more mutually insoluble or sparingly soluble liquids. One of the liquids is usually present in excess and is termed the continuous or external phase, while the liquid dispersed in the continuous phase is termed the dispersed, discontinuous or internal phase. If the continuous phase consists of water, and the dispersed phase consists of an organic liquid, such as mineral oil, the term oil-in-water (O/W) emulsion or suspension is used. If water is finely dispersed in an organic or non-aqueous liquid, a water-in-oil (W/O) emulsion or suspension is produced. If two organic liquids are emulsified in each other, the term oil-in-oil (O/O) emulsion or suspension is used. The term emulsion generally refers to particles less than 1 micrometer (μm) in diameter while the term suspension is usually used to describe particles that are greater than 1 μm in diameter. However, the terms emulsion and suspension are used herein interchangeably to refer to multi-phase systems in which the size of the dispersed phase can range both less than 1 μm and greater than 1 μm.
While O/W and W/O emulsions containing a non-polar oil such as silicone are common, O/O emulsions in which both phases are essentially non-polar are relatively rare. However, the Journal of Colloid and Interface Science, Volume 195, Pages 101-113, Article No. CS975158, Jan. 1, 1997, describes certain paraffin oil-in-silicone oil O/O emulsions, as well as certain silicone oil-in-paraffin oil O/O emulsions. Similarly, emulsions of castor oil in silicone oil, as formulations for drug delivery is described in the Journal of Drug Deliver Science and Technology (2004), 14(2), 113-117.
US Patent Pub. No. 20040002429 describes lubricant compositions comprising an emulsion comprising a low viscosity, relatively non-polar, hydrocarbon carrier fluid and a minor amount of an immiscible or semi-miscible polar, hydrocarbon fluid.
PCT Appl. WO2003/000396 A1 describes emulsions comprising silicones, as either the continuous phase or the dispersed phase, and stabilized by graft and block copolymers, which emulsions are useful for cosmetic applications.
U.S. Pat. No. 6,080,394 A discloses a non-aqueous polar solvent-in-oil emulsion composition containing a non-aqueous polar solvent phase dispersed in a silicone oil continuous phase by an emulsifier. U.S. Pat. No. 6,238,657 B1 describes O/O emulsions, stabilized with silicone elastomers, where one of the oil phases is a silicone oil, while the other oil phase is an organic oil such as mineral oil or castor oil. Also described are three-phase aqueous emulsions derived from such emulsions and their use in personal health care applications.
The formation of O/O emulsions in aliphatic hydrocarbons or the like such as dodecane, which have low dielectric constants, is generally not trivial, especially when certain properties are desired for the two phases in such emulsions. In general, in the formation of emulsions, a stable dispersion of droplets or particles results when the attractive potential between two droplets is less than the repulsive potential. As repulsive potential is directly proportional to the dielectric constant of the dispersion medium, stable dispersions cannot be easily achieved in a medium of very low dielectric constant such as aliphatic hydrocarbons.
Another issue with which to contend, in the case of particles dispersed in low density hydrocarbon solvents such as dodecane is settling of the dispersed phase with time as governed by Stoke's Law that defines settling velocities of particles in a fluid by the following equation:V=(2gr2)(d1−d2)/9μwhere V=velocity of settling, g=acceleration due to gravity, r=radius of particle or dispersed phase, d1=density of dispersed phase, d2=density of medium, and μ=viscosity of the continuous phase. The issue of settling or creaming of particles is especially relevant to electro-optical modulating display devices in which particles are dispersed in a liquid system, such as electrophoretic, electrowetting, or electrochromic display devices. It is important that the particles in such systems remain neutrally buoyant, neither settling nor creaming. Since viscosity and density mismatches of solid particles and the continuous phase in such fluids are usually large, techniques such as increasing the viscosity of the continuous phase using polymeric additives have been employed to overcome this effect, although such solutions can cause the electrical mobility of the particles to be compromised. Another issue, in the case of using silicone oils as the dispersed phase, is that the additives that can be solubilized or dispersed effectively in them, for many such applications, can be limited.
Many of the aforementioned patents and other publications disclose O/O particles that have a fairly broad particle size distribution and none of them disclose particles that can be greater than 1 μm and possess a narrow particle size distribution at the same time.
The use of solid colloidal silica as a suspending agent in stabilizing oil droplets greater than 1 μm in an aqueous medium (O/W) has been described by Wiley et al in U.S. Pat. No. 2,932,629. The stabilization is promoted with a water-soluble “promoter” that affects the hydrophobic-hydrophilic balance of the solid colloidal silica particles. As stated in this patent, the promoter drives the particles of the solid colloid to the liquid-liquid interface between the oleophilic or hydrophobic monomer droplets and the aqueous medium.
In view of the above, therefore, there is a need for an O/O composition in which both the continuous phase and the dispersed phase have certain desirable properties, or combinations of properties, which cannot be obtained with prior-art O/O emulsions such as those in which silicone oil is one of the phases. Among the properties which may be desired for both or more of the emulsion phases, depending on the application, is a low dielectric constant or non-polarality. In addition, it may be desired that the dispersed phase be capable of readily incorporating, into the dispersed phase, colorants, polymers, or other additives. It may also be desirable that the dispersed phase comprises droplets having a size greater than 1 μm. O/O compositions with new or improved properties would be advantageously useful in a variety of applications known to the skilled artisan for such materials. In addition, O/O compositions with new or improved properties, not heretofore obtained, would offer the opportunity for the development of new applications for such materials.